CHAPTER XVII. 



Class HEMICHORDA or ENTEROPNEUSTA. 



Type Balanoglossus. 



A species of Balanoglossus was described by Delle Chiaje 

 at the end of the eighteenth century, but it is only within 

 the last few years that the researches of Spengel, Bateson, 

 and others have led to an appreciation of the importance of 

 the type, and to a recognition of its peculiar features. 



The class (Enteropneusta) which was erected for the 

 reception of Balanoglossus has at present included in it a 

 few other forms, whose more or less distinct affinities with 

 Vertebrates are suggested by the alternative title Hemi- 

 chorda. Taken along with Tunicates and Amphioxus, 

 they illustrate gradual approximations towards Vertebrate 

 characters. 



General Characters. 



The body is divisible into three regions— a pre-oral 

 "proboscis," a " collar " around and behind the mouth, and 

 a trunk, the anterior part of which bears gill-slits. A dorsal 

 nerve-cord arises from the epiblast along the middle line, and 

 is connected, by a ring round the pharynx, with a ventral 

 cord. In the skin, which is covered with ciliated ectoderm, 

 there is also a nerve plexus. From the anterior region of the 

 gut a diverticulum grows forward for a short distance, becomes 

 a solid support for the proboscis, and is often called the 

 " notochord." The gill-slits open dorsally, are very numerous, 

 and increase in number during life; in some details of 

 development they recall those of Amphioxus. The mesoblast 



